1. Field of the Invention
The present invention relates to a method and an apparatus for encrypting and transferring electronic mails, which realize an information management and a transfer control regarding encrypted electronic mails.
2. Description of the Background Art
In conjunction with spread of computer networks, electronic mails are becoming widely used. In addition, an electronic mail encryption apparatus has been developed to meet demands for transferring secret information in forms of electronic mails.
Conventionally, the electronic mail encryption apparatus has been realized in schemes such as PEM (Privacy Enhanced Mail) and PGP (Pretty Good Privacy). In the following, a conventional procedure for encrypting electronic mails will be described.
First, an encryption key of the secret-key cryptography is generated, and an electronic mail message is encrypted by using this encryption key according to the secret-key cryptography. Then, the encryption key used for encrypting message is encrypted by using respective public keys of a sender and a receiver of the electronic mail according to the public-key cryptography, to produce a sender's encryption key information and a receiver's encryption key information. When there are more than one receivers, the encryption key information for each one of these receivers is produced. Then, the encrypted message, a prescribed number of receiver's encryption key information, and the sender's encryption key information are transmitted as an encrypted electronic mail.
When a receiver or a sender wish to read the encrypted electronic mail which has been encrypted in this manner, the encryption key is obtained first by decrypting the encryption key information assigned to himself by using his own secret key. The secret key to be used here is in pair with the public key used in producing the encryption key information, and only a person who has this secret key can obtain the encryption key by decrypting the encryption key information. After the encryption key is obtained, the message can be decrypted by using this encryption key according to the secret-key cryptography so that it becomes possible to read the message of the electronic mail.
Now, in organizations such as corporations and companies, there are demands for realizing the information management regarding contents of the electronic mails, in order to prevent a leak of secret information to outsiders by means of the electronic mails.
In a case of the electronic mail in which the message is not encrypted, its content can be read by anyone other than the sender and the receiver, so that the information management can be realized by preserving copies of the electronic mails as a log in an apparatus such as an electronic mail transfer apparatus.
However, in a case of the conventional encrypted electronic mail, only the sender and the receiver of this electronic mail can decrypt the encrypted message. Consequently, even when a log preserved in the electronic mail transfer apparatus is checked, the content of the encrypted electronic mail cannot be read by an electronic mail manager, so that it has been impossible to realize the information management.
Moreover, the conventional electronic mail transfer apparatus simply carries out a transfer of the electronic mail by determining intended receivers from a header of the electronic mail upon a request for a transfer of the electronic mail. Consequently, even an encrypted electronic mail which has a highly classified information will be transferred if there is a request for a transfer, so that there remains a possibility of an erroneous transfer, which poses a serious problem from a viewpoint of security.
Thus, the conventional electronic mail encryption apparatus has been associated with a problem that the information management cannot be realized because the electronic mail manager cannot read the content of the electronic mail encrypted by this apparatus.
In addition, the conventional electronic mail transfer apparatus has been associated with a problem from a viewpoint of security because the electronic mail has been transferred unconditionally.